Biometric imaging systems include, but are not limited to, print ridge pattern or print imaging systems. These print imaging systems are often referred to in the relevant art as scanners or live scanners. Conventional live scanners use light to detect an image of a print. For example, an object having a print such as one or more fingers can be placed on a platen of a live scanner. An illumination source illuminates the underside of the platen. An image representative of the ridge pattern of the print is detected by an image sensor such as, for example, a solid-state camera.
Increasing demands are placed on the quality of a print image detected by live scanners. Live scanners are desired that can capture print images having a high-contrast and a high-resolution. One standard for live scanners promulgated by the Federal Bureau of Investigation (FBI) is the Integrated Automated Fingerprint Identification System (IAFIS) Image Quality Specifications (IQS) (Appendix F). To gain certification under Appendix F, a live scanner among other things must be able to capture an image at a resolution of 500 dots per inch (dpi) or greater and have generally uniform gray shading across a platen scanning area.
As illustrated by FIGS. 1A and 1B, conventional live scanners have illumination systems that directly illuminate the underside of a platen. As described below, this direct illumination of the underside of the platen has drawbacks.
FIG. 1A illustrates a first conventional live scanner 100 having an optical axis (OA) 101. Live scanner 100 includes an illumination source 102, an illumination optic system 104, a prism 106, a camera optical system 108, and a camera 110. As shown in the cross-sectional view of FIG. 1A, prism 106 includes three surfaces 120, 122 and 124. Surface 120 is the platen (platen 120). As described below, rays of light emitted from illumination source 102 directly illuminate platen 120.
In operation, an object having a print ridge pattern such as, for example, finger 112 is placed on platen 120 of live scanner 100 for imaging. Illumination source 102 of live scanner 100 emits rays of light. These rays of light are gathered by illumination optic system 104 and directed toward surface 122 of prism 106.
Rays of light emitted by illumination source 102 enter prism 106 at surface 122 and travel internally through prism 102 until they hit platen 120. Generally speaking, the rays of light hitting platen 120 are in total internal reflection (TIR). Incident rays of light hitting platen 120 will typically undergo TIR off platen 120 if they hit platen 120 at an angle greater than the critical angle. The critical angle is measured between an incident ray of light and a normal line to platen 120. TIR is broken, however, where a ridge 114 of finger 112 touches platen 120. TIR is not broken by a valley 116 of finger 112.
Rays of light hitting portions of platen 120 corresponding to valleys 114 of finger 112 are totally internally reflected toward surface 124 of prism 106. These totally internally reflected rays of light exit prism 106 at surface 124. Camera optics system 108 collects the rays of light exiting prism 106 at surface 124 and focuses them on an imaging portion of camera 110. Camera 110 forms an image of the print ridge pattern of finger 112 using the totally internally reflected rays of light.
In the image formed by camera 110 of live scanner 100, dark lines correspond to ridges 114 of finger 112 while light lines correspond to valleys 116 of finger 112. As will be understood by persons skilled in the relevant arts, in order to obtain a high-contrast print image using live scanner 100, it is necessary to achieve a flat, uniform illumination of the scanning area of platen 120. This is not a simple task. To accomplish this task, a complex illumination system is often used, which can make live scanner 100 cost prohibitive.
FIG. 1B illustrates a second conventional live scanner 150. Live scanner 150 includes an illumination source 102, a prism 152, a camera optic system 108, and a camera 110. As shown in the cross-sectional view of FIG. 1B, prism 152 includes four surfaces 154, 156, 158, and 160. Surface 154 is the platen (platen 154). Typically, a black coating of paint 170 is applied to surface 158. As described below, rays of light emitted from illumination source 102 directly illuminate the underside of platen 154.
As illustrated in FIG. 1B, in operation, an object having a print ridge pattern such as, for example, finger 112 is placed on platen 154 of live scanner 150 for imaging. Illumination source 102 of live scanner 150 emits rays of light that enter prism 152 through surface 156. These rays of light directly illuminate the underside of platen 154. A portion of these direct rays of light are diffused and scattered by print ridges 114 of finger 112. These diffused and scattered rays of light are used to form a print image. Any direct rays of light from illumination source 102 that do not hit a ridge 114 of finger 112 exit prism 150 at platen 154 because these rays of light are not in TIR. Rays of light exiting prism 150 at platen 154 (e.g., because of the presence of a valley 116 of finger 112) cannot be used to form the print image.
A portion of the rays of light from illumination source 102 that are diffused and scattered by ridges 114 of finger 112 travel directly to camera optics system 108 and camera 110 where they are used to form a print image. Where the fingerprint valleys 116 occur, camera 110 and camera optical system 108 image black painted surface 158 and appear dark with respect to the fingerprint ridges 114. This is because TIR is not broken by a valley 116 of finger 112.
In the print image formed by live scanner 150, light lines correspond to print ridges while dark lines correspond to print valleys. As will be understood by persons skilled in the relevant arts, the reflected rays of light that help form the print image have a lower intensity than the rays of light that travel directly from ridges 114 toward camera optics system 108 and camera 110 without being reflected. This difference in intensity provides image contrast, i.e., a contrast between print ridges and print valleys. The direct rays of light have a higher intensity that the reflected rays of light and thus form the light lines that correspond to print ridges. The lower intensity reflected rays of light form dark lines that correspond to print valleys.
Because live scanner 150 relies on print ridges to diffuse and scatter light used for print image formation, live scanner 150 cannot be used to form high-contrast print images for certain individuals having dark print ridges (i.e., print ridges that contain more than some threshold amount of the skin pigment melanin, particularly the form know as eumelanin). Dark print ridges absorb more light than lighter print ridges. Thus, as a result, dark print ridges diffuse and scatter less light than lighter print ridges, resulting in the formation of lower contrast images.
What is needed is a live scanner that does not have the shortcomings of a conventional live scanner. In particular, what is needed is a live scanner without a complex illumination system that can produce a high-contrast print image for any individual.